That's not true -- VISITING Cuba is NOT a crime. At one point, they tried to make it one -- my oldest passports contain a warning that they are not valid for travel to North Vietnam, North Korea, Mainland China or Cuba (bizarrely, of course, visiting the USSR was never banned). That travel ban was successfully challenged in court as inconsistent with the Constitutionally-guaranteed right to "freely assemble", so the government then announced that it acknowledged the right of citizens to travel to Cuba, but then said that spending any money there would be prosecuted under the WW I-era "Trading with the Enemy" Act. I think most people view this as a slippery bit of legal sleight-of-hand, and apparently the government agrees because any time someone has tried to challenge the governments use of this legislation, the government caves in and drops prosecution; in this way, they avoid having the courts rule against them and prevent them from using the threat of prosecution agaist visitors to Cuba in the future. It's the same legal tactic they used for years with the 'no-fly' list -- every time a plaintiff was on the verge of getting an injunction ordering the government to let them fly, the government suddenly removed them from the list and then argued that the plaintiff had "no standing" because they were free to fly, thereby avoiding a legal precedent that would shut down the 'no-fly' list entirely.

So with respect to Cuba, American travellers who are known to have visited Cuba are routinely subjected to questions about "who paid for your trip?" Unfortunately, the majority of them allow themselves to be browbeaten into making some admission that they spent money there, and it is this self-incrimination that is used to asses a fine against the traveller.

So you are partly correct -- if you go to Cuba, you must list it on your form or risk prosecution for perjury. My routine response to any and all questions posed by US customs and immigration is that "I decline to orally amend the written customs declaration you have in front of you, which I fully and truthfully completed and signed under penalty of perjury" As i have posted elsewhere in this forum, I have had customs agents make all sorts of false and misleading statements about their authority at the border ("the Constitution doesn't apply here because you haven't entered the United States yet"; hmm, yes, well "if I 'haven't entered the United States,' then how is it you have any authority here?"), but the fact is the 5th Amendment protection against self-incrimination, along with the rest of the US Constitution, the United States Code and all treaties etc to which the USA is a party -- with a VERY small, VERY limited exception to some parts of the 4th Amendment's prohibitions against search without a warrant -- are in FULL force at the border.